Method and apparatus for providing time diversity

ABSTRACT

The invention provides a method and apparatus for transmitting digital signaling formation to a receiver using a plurality of antennas. The invention involves applying a channel code to a digital signal producing one or more symbols. A plurality of symbol copies is made and each copy is weighted by a distinct time varying function. Each antenna which are located at two or more base stations transmits a signal based on one of the weighted symbol copies. Any channel code may be used with the invention, such as a convolutional channel code or block channel code. Weighting provided to symbol copies may involve application of an amplitude gain, phase shift, or both. The present invention may be used in combination with either or both conventional interleavers and constellation mappers.

FIELD OF THE INVENTION

The present invention relates generally to the field of communicationssystems, and particularly to the field of wireless communications, suchas, e.g. cellular radio, from two or more base stations to a givenreceiver wherein the base stations transmit the same signal, but arelative time-varying phase offset is applied between the twotransmitted signals.

BACKGROUND OF THE INVENTION

In wireless communication systems, an information signal is communicatedfrom a transmitter to a receiver via a channel comprising severalindependent paths. These paths are referred to as multipaths. Eachmultipath represents a distinct route an information signal may take intraveling between the transmitter and receiver. An information signalcommunicated via such a channel--a multipath channel--appears at areceiver as a plurality of multipath signals, one signal for eachmultipath.

The amplitudes and phases of signals received from a transmitter throughdifferent multipaths of a channel are generally independent of eachother. Because of complex addition of multipath signals, the strength ofreceived signals may vary between very small and moderately largevalues. The phenomenon of received signal strength variation due tocomplex addition of multipath signals is known as fading. In a fadingenvironment, points of very low signal strength, or deep fades, areseparated by approximately one-half of a signal wavelength from eachother.

Wireless communication channels can be described by certain channelcharacteristics, such as amplitude attenuation and phase shifting. Forexample, the multipaths of a channel may provide different amplitudeattenuations and phase shifts to an information signal communicated froma transmitter to a receiver. These different amplitude and phasecharacteristics may vary due to e.g., relative movement betweentransmitter and receiver, or changes in local geography of thetransmitter or receiver due to movement. Because of the variation ofchannel characteristics, a receiver can experience a signal whosestrength varies with time. This variation is the manifestation of thecomplex addition of multipath signals having time varying amplitudes andphases.

If the characteristics of a multipath channel vary slowly, a receiverexperiencing a deep fade may observe a weak signal for a long period oftime. Long fades are not uncommon in, e.g., indoor radio systems, whererelative movement between receivers and transmitters is slow ornonexistent (often, one of these two is an immobile base station; theother is a mobile device carried by a person). Since the duration of adeep fade in an indoor radio system may be large in comparison to theduration of information symbols being communicated, long bursts ofsymbol errors may occur (due to the weakness of received signal strengthfor an extended period of time).

Space diversity is a classical technique for mitigating the detrimentaleffects of fading, such as error bursts. Space diversity is providedthrough the use of a plurality of antennas at a receiver. If thereceiver antennas are separated by more than a couple of wavelengths,the multipath signals received by the individual receiver antennas areapproximately independent of each other. When several antennas are usedby a receiver, the probability that received signals will yield a deepfade at all antennas simultaneously is small. Thus, signals received bythese antennas may be combined to reduce the effects of fading.

Space diversity, however, is not without its drawbacks. For example,space diversity requires the use of a plurality of widely spacedantennas. For small portable receivers this requirement is problematic.Also, space diversity increases the complexity of a receiver, therebyincreasing its cost.

Time diversity is another technique which has been employed to mitigatethe detrimental effects of fading. Time diversity may be achieved bytransmitting a plurality of copies of an information signal duringdistinct time intervals. These transmission time intervals should beseparated in time so that received signals are subjected to independentfades. Once the plurality of signal copies have been received by areceiver, the independent nature of their fades facilitates avoidance ofthe detrimental effects of fading.

Like space diversity, time diversity also has its drawbacks. Timediversity is predicated on the idea of identical signal transmission atdifferent times. However, the time needed to receive a plurality ofcopies of an information signal presents a delay in the communicationprocess which may be undesirable, if not intolerable.

Time diversity can also be effectively obtained when a channel code isused in conjunction with an interleaver/deinterleaver pair well known inthe art. An interleaver receives a set of consecutive channel coded datasymbols for transmission and rearranges them in, e.g., a pseudo randomfashion. Typically, the number of symbols in the set extend for aduration beyond that of a slow deep fade. Rearranged symbols aretransmitted over the channel to a receiver having a single antenna. Byvirtue of transmission, consecutive symbols are subject to similarfading. However, these consecutively transmitted symbols are not inoriginal order. A receiver equipped with a deinterleaver rearranges thesymbols back to their original order. Due to the randomness of theirtransmission order, data symbols presented to the channel decoder by thedeinterleaver have been subject to essentially independent fades. Theindependent symbol fading afforded by the interleaver/deinterleaver pairmay be utilized to avoid fading's detrimental effects.

However, as with the first time diversity technique discussed above, atransmission delay is created by this approach. This delay is directlyproportional to the size of the interleaver. Allowable transmissiondelay imposes limits on the size of the interleaver. However, aninterleaver of a size beyond the imposed limit may be needed to dealeffectively with fading.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for mitigating thedetrimental effects of fading. It does this by effectively varying thecharacteristics of a multipath communication channel to provide a timediversity with a reduced delay effect. More specifically, this inventionprovides for the wireless transmission from two or more base stations toa given receiver. The base stations transmit the same signal, but arelative time-varying phase offset is applied between the twotransmitted signals. This phase offset avoids long slow fades and theproblems associated therewith.

A first illustrative embodiment of the present invention provides timediversity by increasing the rate of multipath channel fading. Theembodiment further provides information redundancy through use of achannel code. The increased fading rate shortens the duration of fadesso as to facilitate avoidance of long error bursts. The redundancyintroduced by the channel code helps mitigate errors which may occur dueto fading. The embodiment provides a channel coder for applying thechannel code to a digital information signal. The channel coder producesone or more coded information symbols which are processed by aconstellation mapper. A copy of each symbol is then provided to amultiplier employed in each of the transmitters. Each multiplier isassociated an antenna at each transmitter. Each multiplier weighs thecopy of the symbol with a distinct time varying function.Illustratively, each of these time varying functions provides a distinctphase offset to a copy of a symbol. The output of each multiplier isprovided to its associated antenna at each transmitters for transmissionto a receiver. The receiver for use with this embodiment comprises asingle antenna for receiving the weighted symbols and a channel decoderwhich is complementary to the channel encoder.

A second illustrative embodiment of the present invention employs aparticular kind of channel code, referred to as a block code. Like thefirst embodiment, this embodiment employs two or more base stations eachhaving an antenna. The base stations transmit the same signal, but arelative time-varying phase offset is applied between the twotransmitted signals. This phase offset avoids slow fades. Each basestation employs multipliers such as those described above to weight eachof M copies of a block coded symbol, this time with a distinct discretephase shift where M is a positive integer representing the number ofbase stations. Each weighted copy is provided for transmission to areceiver by an antenna. As with the first embodiment, a receiver for usewith this embodiment comprises a single antenna for receiving theweighted symbols and a channel decoder which is complementary to thechannel encoder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B presents two signal phasors from two transmittingantennas at two base stations at specific points in space where deepfades occur.

FIG. 2A and 2B presents a first illustrative embodiment of the presentinvention.

FIG. 3 presents a receiver for use with the first illustrativeembodiment.

FIGS. 4A and 4B present the illustrative embodiments of FIGS. 2A, 2B and3 augmented to include an interleaver/deinterleaver pair, respectively.

FIGS. 5A and 5B present a second illustrative embodiment of the presentinvention.

FIG. 6 presents a receiver for use with the second illustrativeembodiment.

DETAILED DESCRIPTION Introduction to the Illustrative Embodiments

The illustrative embodiments of the present invention concern a wirelesscommunication system such as, e.g., an indoor radio communicationsystem, a cellular system, or personal communications system. In suchsystems, two or more base stations commonly use one or more antennas forreceiving transmitted signals. These antennas provide the base stationwith space diversity. According to the principles of the presentinvention, each antennas at each base station should be used for thetransmission of signals to the mobile units. Accordingly, two or morebase stations may be utilized in order to transmit the wirelesstransmission to a given receiver. Advantageously, the same plurality ofantennas used for base station reception may be used for transmission tothe mobile units. These mobile units employ but one antenna.

Consider, for example, an indoor radio system comprising a first basestation B1 having an antenna, T1, and a second base station B2 having anantenna, T2, both antennas T1 and T2 are provided for transmitting achannel coded signal in, for example, a Rayleigh fading channel (thatis, a channel without a line-of-sight path between transmitter andreceiver) and a mobile receiver. In such a system, the typical delayspread between received multipath symbols is on the order of severalnanoseconds-a very small spread in comparison to the duration of achannel code symbol.

FIG. 1 depicts received signal phasors S1 and S2, from antennas T1 andT2 at respective base stations B1 and B2, at specific points in spacewhere a deep fade can occur. The signals S1 and S2 are independently andidentically distributed with, e.g., Rayleigh amplitude and uniformphase. Furthermore, the characteristics of the channel through whichphasors S1 and S2 are communicated change slowly, so that the deep fadesdepicted in FIG. 1 are essentially static. The deep fades at thelocations corresponding to FIG. 1(a) occur because of destructiveaddition of the signals from the two base station antennas. The deepfades shown in FIG. 1(b) occur because of the weakness of receivedsignal energy from each individual antenna T1 and T2.

The first illustrative embodiment of the present invention introducesvery small time varying phase offsets θ 1(n) and θ 2(n) to the signalstransmitted at antennas T1 and T2, respectively. These offsets have theeffect of a slow rotation of the phasors S1 and S2. If θ 1(n) and θ 2(n)are time varying, S1 and S2 will destructively interfere with each otheronly for a small fraction of time. If a channel code is employed, thistechnique can be used to reduce the deep fades shown in FIG. 1(a).

The first illustrative embodiment may be extended to deal with the deepfades presented in FIG. 1(b). All that is required is the use ofadditional transmitting antennas to help contribute to received signalstrength. A discussion of the embodiment below is generic to the numberof transmitting antennas, each located at base stations B1 and B2. Itwill be appreciated that an embodiment of the invention may be employedwith M base stations having an antenna that transmits appropriatesignals to avoid deep fades as illustrated herein.

Like the first illustrative embodiment, the second illustrativeembodiment introduces phase offsets prior to signal transmission. In thesecond embodiment, a particular type of channel code-a block code-isused. Given the use of this code, the phases of signals transmitted fromone or more of the base station and their antennas are shifted to takeon a set of discrete values which depend on the number of base stations,M, and the length of a block code codeword, N.

As with the first illustrative embodiment, this embodiment addressesboth types of deep fades presented in FIG. 1, with the deep fade shownin FIG. 1(b) addressable by extension to a larger number of transmittingantennas. A disclosure of this embodiment is also generic to the numberof antennas provided, M, and the number of symbols in a codeword, N.

For clarity of explanation, the illustrative embodiments of the presentinvention are presented as comprising individual functional blocks. Thefunctions these blocks represent may be provided through the use ofeither shared or dedicated hardware, including, but not limited to,hardware capable of executing software. Illustrative embodiments maycomprise digital signal processor (DSP) hardware, such as the AT&T DSP16or DSP32C, and software performing the operations discussed below. Verylarge scale integration (VLSI) hardware embodiments of the presentinvention, as well as hybrid DSP/VLSI embodiments, may also be provided.

A First Illustrative Embodiment

A first illustrative embodiment of the present invention is presented inFIGS. 2A and 2B. The embodiment are radio communication system basestation transmitter B1 and B2 for use in, e.g., cellular radio and othertypes of personal communications systems. As is evident, two or morebase stations are utilized to transmit the wireless transmission to agiven receiver. As shown in FIGS. 2A and 2B, each base station B1 and B2transmits the same signal, but a relative time-varying phase offset isapplied between the two transmitted signals.

As shown in FIGS. 2A and 2B, each transmitter B1, B2 comprises a channelcoder 20, a DPSK modulator 30, multiplication circuit 50, transmissioncircuitry 52 (comprising conventional carrier, pulse shaping, and poweramplification circuits), and a transmitting antenna 55.

Channel coder 20 may be any of the conventional channel coders wellknown in the art. These include convolutional or block codes, e.g., arate one-half memory 3 convolutional code. Coder 20 provides a channelcode to a pulse code modulated digital information signal, x(i),representing, e.g., speech. Signal x(i) is provided by a conventionalinformation source 10 such as, e.g., an ordinary telephone network,coupled to the base station transmitter, providing signals to betransmitted to a mobile receiver or, more simply, a microphone, audiofront-end circuitry, and an analog-to-digital converter in combination.It will be apparent to one of ordinary skill in the art that the presentembodiments may be used with any information source which provides ormay be adapted to provide digital data.

Output from channel coder 20 are complex data symbols, a(n), wherea(n)=a_(r) (n)+j a_(i) (n) and n is a discrete time index;illustratively, a_(r) (n), a_(i) (n)ε -1,1! (the discrete time index ihas been changed to n to reflect the fact that the time indices forinformation bits and channel coded symbols may not coincide). Each suchsymbol, a(n), is provided to a conventional 4-DPSK constellation mapper30 well known in the art. Constellation mapper 30 comprises aconventional Gray coded 4-PSK constellation mapper, a multiplier 35, anda unit delay register 37. It will be further apparent to one of ordinaryskill in the art that any conventional constellation mapper, or noconstellation mapper at all, may be used with these embodiments.

The 4-PSK constellation mapper 30 processes complex data symbols a(n)received from channel coder 20 as follows: ##EQU1##

The Gray coded 4-PSK complex symbols, α (n), are provided tomultiplication circuit 35 where they are multiplied by the output ofunit delay register 37 as follows:

    u(n)=α(n)u(n-1)                                      (2)

The results of the operation of the multiplication circuit 35 and delayregister 37 are complex 4-DPSK coded symbols, u(n). This embodimentprovides one complex 4-DPSK symbol, u(n), for each complex symbol, a(n),provided by the channel coder 20. Each such symbol, u(n), is provided toa multiplication circuit 50, transmission circuitry 52, and transmittingantenna 55. Illustratively, there may be two base stations.

Each multiplication circuit 50 multiplies a complex symbol, u(n),provided by constellation mapper 30, by a complex time-varying functionof the form

A_(m) (n)e^(j).spsp.θm(n)

where m indexes the plurality of M antennas, each associated with a basestation, A_(m) (n) is an amplitude weight for the mth antenna, and θ_(m)(n) is a phase offset for the mth antenna.

Illustratively, ##EQU2## where f_(m) =f.sub.Δ (m-1)-1/2(M-1)!; T_(d) isthe reciprocal of the transmitted symbol data rate; and f.sub.Δ is asmall fraction of the transmitted symbol date rate, e.g., 2% thereof.So, for example, if the data symbol rate is 8 kilosymbols/second, T_(d)=0.125 ms and f.sub.Δ =160 Hz (symbols/second).

What results from the operation of multiplier 50 is a complex symbol,c_(m) (n), provided to a conventional transmission circuitry 52 and anantenna 55. Signals reflecting symbols c_(m) (n) are transmittedsubstantially simultaneously by antennas 55 to a conventional singleantenna receiver equipped with a channel decoder (complementary tochannel coder 20). Thus, the first illustrative embodiment provides forparallel transmission of data symbols by a plurality of B base stationsand wherein each symbol is multiplied prior to transmission by a uniquecomplex function.

FIG. 3 presents an illustrative conventional receiver for use with thefirst illustrative embodiment presented in FIG. 2. The receivercomprises an antenna 60 and conventional front-end receiver circuitry 62(comprising, e.g., low noise amplifiers, RF/IF band-pass filters, and amatch filter) for receiving a transmitted signal, s(n), from thetransmitting antennas 55. Signal s(n) is given by the followingexpression: ##EQU3## where M is the total number of transmittingantennas 55, A_(m) (n) and θ_(m) (n) are as described above, β_(m) (n)represents the complex fading on each of m multipath channels, u(n) isas described above, and v(n) is a complex additive white Gaussian noisecomponent (of course, expression (4) is merely a model for a signalactually received by the receiver; no calculation of expression (4) isneeded for purposes of the present invention).

Signal s(n) is provided to a 4-DPSK demodulator 65. The output of the4-DPSK demodulator 65, a(n), is an estimate of the output of the channelencoder 20 of the transmitter (the "0" indicating an estimated value).Demodulator 65 provides a(n) according to the following expression:##EQU4## where s* indicates the complex conjugate of s. Complex symbola(n) is then provided to conventional channel decoder 70 (complementaryto channel coder 20) which provides a decoded information signalestimate, x(i). Information signal estimate, x(i), is provided toinformation sink 75 which makes use of the information in any desiredmanner, such as, e.g., digital-to-analog conversion, amplification andapplication to a transducer such as a loudspeaker.

The first illustrative embodiment of the present invention may beaugmented to include a conventional interleaver/deinterleaver pair. SeeFIG. 4A and 4B. As noted previously, use of an interleaver/deinterleaverpair in conventional slowly fading systems can result in largetransmission delays. This is because to be useful, an interleaver mustoperate on many symbols (i.e., a number of symbols which when multipliedby the reciprocal of the symbol data rate yields a duration far inexcess of the duration of an expected fade). For example, assuming aconvolutional channel code, the interleaver operates on a number ofsamples equal to ten times the duration of an expected fade. Thus, aconventional deinterleaver must wait to receive all such symbols beforedeinterleaving can occur. This causes delay.

By virtue of the faster fading provided by the first illustrativeembodiment of present invention, a smaller interleaver/deinterleaverpair may be used, resulting in enhanced performance with less delay thanthat associated with slowly fading channels.

The first illustrative embodiment of the present invention may beadvantageously combined with conventional multiple antenna receiversproviding space (or antenna) diversity. All that is required of thereceiver is that it employ a channel decoder which is complementary tothat used in the transmitter.

A Second Illustrative Embodiment

The second illustrative embodiment of the present invention is presentedin FIGS. 5A and 5B. As with the first illustrative embodiment of thepresent invention, each base station includes an information source 10which presents a digital information signal, x(i), for transmission to areceiver. The channel coder 85 provides an illustrative block code-e.g.,a conventional one-half rate repetition code with block length N=2. Therepetition code provided by the channel coder 85 produces a code symbold(n)ε -1,1!. This symbol is repeated such that the output from coder 85at time n comprises two symbols, a₁ (n) and a₂ (n), both of which areequal to d(n).

The coded symbols, a₁ (n) and a₂ (n), are mapped to a 4-PSKconstellation using a Gray coder 90. The output of Gray coder 90 isprovided according to the following expressions: ##EQU5##

This output, α (n), is provided to multiplier 35 where it is weighted bythe output of two symbol unit delay 95. This weighting provides 4-DPSKconstellation mapping 97 according to the following expression:

    u(n)=α(n)u(n-2)                                      (8)

For each base station, the result of 4-DPSK constellation mapping, u(n),is provided to a multiplier 50, transmission circuitry 52, andassociated antenna 55.

As a general matter, for a block code wherein each codeword comprises Nsymbols (with time indices given by n, n+1, . . . , n+N-1), eachmultiplier provides a phase shift, θ_(m), for the signals to betransmitted at an antenna at the Mth base station. For up to the first Msymbols of a codeword, the phase shift θ m applied by the mth multiplieris: ##EQU6## For any symbols of a codeword exceeding M (i.e., M<N), thephase shift θ_(m) (1) applied by the mth multiplier is: ##EQU7## where 1is a time index and k and n' are integers which satisfy the followingexpressions:

    (k-1)M+1≦N≦kM

and

    n'(k-1)<(1-M-n+1)≦(n'+1)(k-1)                       (11)

This phase shift technique provides M uncorrelated symbols and N-Mpartially decorrelated symbols. As with the first embodiment of thepresent invention, this embodiment may be augmented with a conventionalinterleaver/deinterleaver pair. In this case, theinterleaver/deinterleaver pair operates to further decorrelate the N-Mpartially decorrelated transmitted symbols.

For the above second illustrative embodiment, the first multiplier 50 atthe first base station provides a phase shift θ₁ (n)=θ₁ (n+1)=0, whilethe second multiplier 50 at the second base station provides a phaseshift θ₂ (n)=0 and θ₂ (n+1)=π. Each of these multipliers provides anamplitude A m (1) of unity. The signal received by a receiver, s, willbe of the form:

    s(n)=(β.sub.1 (n)+β.sub.2 (n))u(n)+v(n)

    s(n+1)=(β.sub.1 (n+1)-β.sub.2 (n+1))u(n+1)+v(n+1)(12)

where β₁ (n) and β₂ (n) are the complex fading coefficients, T is thesampling interval, and v(n) is the additive white Gaussian noisecomponent. The fading coefficients β₁ (n) and β₂ (n) are independentlyand identically distributed complex Gaussian random variables with meanequal to zero.

The complex envelope of the received signal, s, is

    r(n)=β.sub.1 (n)e.sup.jθ.sbsb.1.sup.(n) +β.sub.2 (n)e.sup.jθ.sbsb.2.sup.(n)                          (13)

For this embodiment, values r(n) and r(n+1) are completely uncorrelated.

FIG. 6 presents an illustrative receiver for use with the secondillustrative embodiment presented in FIG. 5. The receiver comprises anantenna 60 and conventional front-end receiver circuitry 62 forreceiving a transmitted signal, s(n), from the transmitting antennas 55of each base transmitter B1 and B2. Signal s(n) is given by (12).

Signal s(n) is provided to the 4-DPSK demodulator 100. The output of the4-DPSK demodulator 100, a 1,2 (n), is an estimate of the output of thechannel encoder 85 of the base transmitters (the "0" indicating anestimated value). Demodulator 100 provides z(n) according to thefollowing expression: ##EQU8## where s* indicates the complex conjugateof s. Complex symbol z(n) is further processed by demodulator 100 toprovide values for a 1,2 (n) as follows:

    a.sub.1 (n)=Re z(n)!;

    a.sub.1 (n+1)=Im z(n)!;

    a.sub.2 (n)=Re z(n+1)!;                                    (15)

    a.sub.2 (n+1)=Im z(n+1)!;

Values of a₁,2 (n) are then provided to channel decoder 10(complementary to channel coder 85) which provides a decoded informationsignal estimate, x(i) by

(i) forming a value U(n)=a₁ (n)+a₂ (n);

(ii) determining a(n) as follows:

    U(n)>0d(n)=1

    U(n)<0d(n)=-1;                                             (16)

and

(iii) conventionally decoding d(n) to provide x(i). Information signalestimate, x(i), is provided to information sink 75 which makes use ofthe information in any desired manner.

The above discussion of the second embodiment includes an example whereN,M=2. The generality of equations (9)-(11) may further be seen whenN>M. For example, assuming, N=4 and M=2, the first multiplier 50provides no phase shift (i.e., u(n) multiplied by unity), and the secondmultiplier provides phase shift for each block of θ₂ (n)=0; θ₂(n+1))=π;θ₂ (n+2)=π/2; and θ₂ (n+3)=3π/2. In this case, the signalreceived by a receiver, s, will be of the form:

    s(n)=(β.sub.1 (n)+β.sub.2 (n))u(n)+v(n)

    s(n+1)=(β.sub.1 (n+1)-β.sub.2 (n+1))u(n+1)+v(n+1)

    s(n+2)=(β.sub.1 (n+2)+jβ.sub.2 (n+2)) u(n+2)+v(n+2)

    s(n+3)=(β.sub.1 (n+3)-jβ.sub.2 (n+3)) u(n+3)+v(n+3)(17)

The complex envelope of the received signal is

    r(n)=(β.sub.1 (n)e.sup.jθ.sbsb.1.sup.(n) +β.sub.2 (n)e.sup.jθ.sbsb.2.sup.(n)                          (18)

According to this particular embodiment (where N=4 and M=2), r(n) andr(n+1) are uncorrelated as are r(n+2) and r(n+3). Values r(n+2) andr(n+3) are partially decorrelated from r(n) and r(n+1). Use of aconventional interleaver/deinterleaver pair will render all four ofthese values approximately uncorrelated. In a very slow fading channelin the absence of phase variations provided in accordance with thepresent invention, all four of these values, r(n), r(n+1), r(n+2), andr(n+3), will be highly correlated and will require an interleaver oflarge dimension. By use of the present embodiment, only two of the fourvalues need decorrelation by operation of an interleaver. Thus, thedelay due to interleaving may be reduced by more than a factor of two.

I claim:
 1. A method of transmitting digital signal information to areceiver with use of a plurality of M antennas, each antenna located atone of a plurality of base stations, at least two of the base stationshaving an antenna thereat, the method comprising the steps of:applying achannel code to a digital signal to produce one or more symbols;providing a copy of a symbol at each base station; weighting each of theM copies of the symbol with a distinct time varying function to form Mweighted symbol copies; and substantially simultaneously transmitting Msignals with M different antennas, each antenna located at a basestation, each transmitted signal based on a distinct one of the Mweighted symbol copies.
 2. The method of claim 1 wherein the step ofapplying a channel code comprises the step of applying a convolutionalcode.
 3. The method of claim 1 wherein the step of applying a channelcode comprises the step of applying a block code.
 4. The method of claim1 wherein each time varying function provides an amplitude gain to asymbol.
 5. The method of claim 4 wherein the amplitude gain is ##EQU9##6. The method of claim 1 wherein each time varying function provides aphase shift to a symbol.
 7. The method of claim 6 wherein a phase shiftapplied to symbols is based upon a transmitted symbol data rate.
 8. Themethod of claim 6 wherein a phase shift applied to an nth symbol for themth antenna is ##EQU10## wherein f₆₆ is a fraction of a transmittedsymbol data rate and Td is a reciprocal of the transmitted symbol datarate.
 9. The method of claim 6 wherein a phase shift, θ_(m) (1), appliedto an nth symbol for the mth antenna is ##EQU11##

    for n≦1≦(n+M-1), ##EQU12##

    for n+M≦1≦(n+N-1),

where k and n' are integers which satisfy the following expressions:

    (k-1)M+1≦N≦kM

and

    n'(k-1)<(1-M-n+1)≦(n'+1)(k-1).


10. The method of claim 1 further comprising the step of processing aplurality of symbols with an interleaver.
 11. The method of claim 1further comprising the step of processing a symbol with a constellationmapper.
 12. The method of claim 11 wherein the constellation mappercomprises a DPSK constellation mapper.
 13. The method of claim 11wherein the constellation mapper comprises a PSK constellation mapper.14. A transmitter for a wireless communication system for transmittingsignals to a receiver, the transmitter comprising:a plurality of basestations, each having a channel coder for receiving a digitalinformation signal and producing one or more symbols based on saiddigital information signal, at least two of said plurality of basestations having an antenna thereat; an information symbol weightingmeans at each of said plurality of base stations, coupled to eachchannel coder, each such means for weighting a symbol with a distincttime varying function; and said antennas coupled to each of said symbolweighting means, for transmitting substantially simultaneously M signalsbased on weighted symbols.
 15. The transmitter of claim 14 wherein thesignals for transmission to a receiver are provided to the transmitterby an information source comprising a telephone network.
 16. Thetransmitter of claim 14 wherein the channel coder comprises aconvolutional channel coder.
 17. The transmitter of claim 14 wherein thechannel coder comprises a block code channel coder.
 18. The transmitterof claim 14 wherein one or more of the M information symbol weightingmeans comprise a multiplier applying the distinct time varying function.19. The transmitter of claim 14 wherein the time varying functionprovides a phase shift to a symbol.
 20. The transmitter of claim 14wherein the distinct time varying function provides an amplitude gain toa symbol.
 21. The transmitter of claim 14 further comprising aconstellation mapper, coupled to receive symbols and to provide mappedsymbols to the plurality of weighting means.
 22. The transmitter ofclaim 21 wherein the constellation mapper comprises a PSK constellationmapper.
 23. The transmitter of claim 21 wherein the constellation mappercomprises a DPSK constellation mapper.
 24. The transmitter of claim 14further comprising an interleaver, coupled to the channel coder, forproducing a plurality of interleaved symbols.